What is the difference between signature_request_ID and signature_ID?

The difference between signature request ID and signature ID are as follows:

Signature request ID: The identification number used for an overall signature request

Signature ID: The identification number used for an individual signer on a signature request.

For example, let's say you send out one signature request with ten signers. In the API response there will be one signature request ID (for the one signature request that was sent) and ten separate signature IDs (one ID for each signer on the request).